A portal, or rift, is doorway that exists between different realms, planes of existence, or universes. They are usually hidden and often guarded with magic and rituals.
It's a tear in space and time... a doorway to another world.
As the Nephilim Jack Kline is close to being birthed, his power surges through his mother and is released, causing a puncture in the fabric of the universe and opening a door to Apocalypse World, an alternate version of Earth. Castiel, Sam, and Dean travel back and forth through it, ultimately using it to trap Lucifer on the other side. The portal is closed due in part to Crowley sacrificing himself as the final ingredient for the ritual to seal the rift. The portal fully closes as Jack is born, just as Lucifer is knocked back into it and pulls Mary through to Apocalypse World with him.
A second portal between the Earths was created with the aid of the alternate universe version of Kevin Tran. Through deciphering the angel tablet, Kevin was able to create a spell to open a new rift using a portion of Lucifer's archangel grace. The spell, however, was designed to admit one person at a time and closed after Lucifer made his escape through it. He landed somewhere in Cincinnati, Ohio.
The Winchesters later learn of a spell from the demon tablet that can open a portal to alternate universes; it requires the blood of a most holy man, fruit from the Tree of Life, the Seal of Solomon, archangel grace, and something that has already been to the world in question to act as a guide.  After gathering all of the ingredients, Sam is able to use the spell to open a twenty-four hour portal identical to the one opened by Jack's birth and it is used by Dean and Ketch. It later closes after the twenty-four hours are over.
After convincing Gabriel to help, the Winchesters, Castiel and Rowena make a failed attempt to open a portal using a small amount of Gabriel's grace. The portal opens but quickly fizzles out. In order to get the portal open and keep it open, they capture Lucifer and drain his grace continuously in order to keep the portal open indefinitely.  After Lucifer is accidentally flung through the portal by Rowena, he leaves behind enough grace to keep it open for another thirty-one hours with Rowena using power from the Black Grimoire to extend that time a little to enable a mass evacuation of dozens of people from Apocalypse World. Once everyone is through, the portal closes and Rowena tells Sam that she couldn't have held it open any longer. Lucifer and Michael later use the same spell to open a portal of their own to reach the Winchesters' world.
Fandom nicknamed the portal the "glowy space vagina."
While trying to create a portal to Apocalypse World, Jack Kline and Kaia Nieves accidentally create a portal to the alternate universe known as the Bad Place by mistake due to Kaia flashing between the Bad Place and Apocalypse World while dreamwalking for Jack. The portal is identical to the one for Apocalypse World and remains open for a few days before closing on its own. 
Dark Kaia is able to open an identical portal from the Bad Place into a Sioux Falls park and come through. She later explains to Dean and Castiel that she opened her rift using magic originating in the Bad Place. However, that same magic doesn't work in the Winchesters' world and she can't use it to open a portal home. Instead, Dark Kaia requests the help of Jack to open a new rift for her to go back to her home.
After learning that the Bad Place is dying due to God ending all of the other worlds, Dark Kaia requests that Jack send her back home, revealing that Kaia is still alive and trapped in the Bad Place. After trying and failing to find another solution, Jack opens a portal to the Bad Place with Merle helping to shield his use of powers from God. This time, Jack is able to open the portal without having to use a dreamwalker, though he had dreamwalked to the Bad Place through Dark Kaia shortly before, possibly negating his need to use a dreamwalker to find the world he is looking for. After Sam and Dean return with a rescued Kaia, the portal closes behind them.
After Metatron expelled all the angels and locked the Gates of Heaven, he created a piece of spell work that allowed him to come and go from Heaven to Earth. The spell simply requires a specific sigil -- Metatron's Cube -- to be drawn. Once done, it creates a doorway into Heaven.
The portal is currently located in a playground. Prior to Metatron's reign coming to end he was known to move the location of the entrance around. It is always guarded by one or two angels, who draw the sigil in a sandbox. As Metatron's spell has not been reversed, even after his capture, this became the only method by which angels can access Heaven from Earth. When the Cosmic Entity invaded Heaven to take Jack's soul to the Empty, he opened all the gates sealed by Metatron. However, the angels continue to use the portal in the playground as they still have no wings.
In the Abrahamic religions, the idea of a "stairway to Heaven" is known as Jacob's ladder.
Uses of the portal:
- 9.23 Do You Believe in Miracles?
- 10.17 Inside Man
- 12.15 Somewhere Between Heaven and Hell
- 12.19 The Future
- 13.07 War of the Worlds
- 13.13 Devil's Bargain
- 13.19 Funeralia
- 14.18 Absence
- 14.19 Jack in the Box
There are a number of Hell Gates, also known as Devil's Gates, which are portals between Hell and Earth. When opened they allow demons passage to Earth. One of the major gates is the Devil's Gate in Wyoming which Samuel Colt built a huge devil's trap around to prevent demons accessing it. There was a church at each point and the sides of the pentacle were made of iron railroad lines. Azazel plan for Lucifer's release included opening the gate to release demons to use in a war against humanity.
As the devil's trap prevents him from getting to the Gate, he uses Jake Talley -- one of the Special Children to cross into it and unlock the gate using the Colt as the key. When the door is open the blast of energy lights up the surrounding railways and blows up at least one of the churches, which significantly weakens the devil's trap and allows Azazel to enter. Hundreds of beings, including demons and spirits, escape from Hell before Bobby and Ellen manage to close the gate. John Winchester is among those to escape through the gate.
There is also a secret "backdoor" entrance to Hell located in Purgatory that is used by rogue reapers as a means of ferrying souls around. The location of the portal can be found by following a specific stream to where three trees meet as one. Where they meet, there are rocks, and between the rocks is the portal.
After Chuck initiated "the End", Belphegor tells Castiel that what God did was not open a door or gate, but cause a tear which ripped Hell open. The rupture opened up in a cemetery near the town of Harlan, Kansas, where billions of souls from Hell began to spill out into the world. On the Earth side, the rupture took the form of a giant chasm in the Earth while in Hell it acted as the celling. While open, the rupture acts as a two-way door, allowing travel both into and out of Hell. The rupture is closed by the Winchesters and Rowena using the Sanetur Acre Vulnus spell, a spell which essentially healed the wound created by the Rupture.
There are a number of portals to Purgatory.
3. Crowley and Castiel torture Eleanor Visyak and find a ritual to open a door to Purgatory. Castiel uses the ritual, which involves drawing a series of sigils in a mixture of the blood of a virgin and the blood of a Purgatory native. A spell is then recited on the night of the lunar eclipse:
Ianua Magna Purgatorii Great door of Purgatory
Clausa Est Ob Nos That is closed for us
Lumine Eius Ab Oculis Its light is kept away
Nostris Retento From our eyes
Sed Nunc Stamus Ad Limen Huius But now we're standing on this threshold
Ianuae Magnae Et Demisse Of the Great door, and humbly
Fideliter Perhonorifice Faithfully, respectfully
Paramus Aperire Eam Are preparing for its opening
Creaturae Terrificae Quarum Ungulae Terrible creature, whose claws
Et Dentes Nunquam Tetigerunt And teeth have never touched
Carnem Humanam Aperit Fauces Human flesh, is opening its jaws
Eius Ad Mundum Nostrum Nunc Toward our world now
Ianua Magna Great door
Aperta Tandem! Open at last!
4. After Death agrees to engineer another lunar eclipse, Dean uses the same ritual and blood to reopen a portal to Purgatory, allowing all the monstrous souls inside Castiel to return - except those of the Leviathans.
6. Apparently in case humans ever accidentally got transported there, God put in a portal that humans could use to escape, but no other monster could use. Benny leads Dean and Castiel to the location of the portal which someone had told him and it opens in reaction to Dean's presence. Carrying Benny's soul in his arm, Dean is able to make it through just before it closes, allowing him to later resurrect the vampire, but Castiel purposely lets go, wanting to stay. It is unclear if an angel would have made it through the portal anyway. Sam is later able to use this portal to escape with the soul of Bobby Singer after briefly getting trapped in Purgatory himself.
7. Angels were able to travel into Purgatory if necessary to rescue beings as they did with Castiel, but this cost the lives of a number of angels. How they were able to enter Purgatory was not revealed.
9. Along with the portal to Earth, a hidden portal exists to Hell that acts as a "backdoor" into the realm. Rogue reapers use this to transport souls between Earth and Hell for a price and Sam uses it to rescue Bobby Singer.
French Mistake World
Balthazar appears gives Sam and Dean a key to an alternate universe where he has stored the stolen angelic weapons of Heaven, which will help Castiel in his fight against Raphael. However, this was revealed to be a ruse. Sam and Dean find a world where their lives are the basis of a TV show, and magic and the supernatural don't exist. The portal is a window in Bobby's house on one side, and a window in the TV set of Bobby's house on the other. 
Access to Oz can be achieved through the use of one of the six keys forged in Oz. If the key is used on any door, said door will turn into a doorway to Oz. Conversely if the key is used in Oz, it will become a doorway to Earth. Entrance to Oz can also be achieved through natural disasters like tornadoes, whirlpools, or the eye of a hurricane.
The Wizard of Oz was also shown to be powerful enough to create a portal from Oz to Earth through the use of magic, using a mirror as a conduit.
Tentacle Porn Land
Dubbed "tentacle porn land" by Dean Winchester, this alternate universe is home to the cosmic deities Yokoth and Glythur, who have consumed and destroyed most of it. A rift between the universes is created by a rogue sect of the Men of Letters led by Diego Avila using a spell that requires the blood of a most holy man, fruit from the Tree of Life, archangel grace, and the Seal of Solomon. 
Nick performs a ritual designed to resurrect Lucifer from the Empty. When completed, a black, viscous portal to the Empty opens, allowing Lucifer to cross over back to Earth. Before Lucifer can possess Nick once more, Jack arrives and sends Lucifer back to the Empty, closing the portal as well.
An alternate universe where John Winchester formed an organization called HunterCorp. As the world was destroyed by God, John and his sons opened a portal and drove through in their car, arriving in Sam and Dean's world. However, John vanished, leaving it unknown if he survived or was sent to a different world altogether. After they came through, God destroyed their world behind them, causing the portal to suck them back in and trap the alternate Winchesters between worlds. Later, the Winchesters, Castiel, and Jack were able to open a portal for their counterparts using a variation of the spell from the demon tablet powered by some of Castiel's grace as the alternate Sam and Dean were already halfway there and thus the portal didn't require nearly as much power. The effort succeeded and the alternate Sam and Dean were brought through to Sam and Dean's world.
- 12.23 All Along the Watchtower
- 13.07 War of the Worlds
- 13.14 Good Intentions
- 13.17 The Thing
- 13.18 Bring 'em Back Alive
- 13.21 Beat the Devil
- 13.22 Exodus
- 13.23 Let the Good Times Roll
- 13.09 The Bad Place
- 13.10 Wayward Sisters
- 15.12 Galaxy Brain
- 9.18 Meta Fiction
- 6.21 Let It Bleed
- 6.12 Like a Virgin
- 6.22 The Man Who Knew Too Much
- 7.01 Meet the New Boss
- 7.23 Survival of the Fittest
- 8.01 We Need to Talk About Kevin
- 8.07 A Little Slice of Kevin
- 8.19 Taxi Driver
- 15.08 Our Father, Who Aren't in Heaven
- 6.15 The French Mistake
- 9.04 Slumber Party
- 10.11 There's No Place Like Home
- 13.17 The Thing
- 14.17 Game Night
- 15.18 Despair
- 15.13 Destiny's Child